CRC Calculation for the Nordic nRF24L01

main.py

"""Small snippet that demonstrates how to calculate the crc for packets transmitted via the Nordic NRF24L01."""


def bin2hex(x):
    def bin2hex_helper(r):
        while r:
            yield r[0:2].upper()
            r = r[2:]

    hex_data = hex(int(x, 2))[2:]
    return list(bin2hex_helper(hex_data))


def split_packet(packet, parts):
    """Split a string of 1s and 0s into multiple substrings as specified by parts.
    Example: "111000011000", (3, 4, 2) -> ["111", "0000", "11", "000"]
    :param packet: String of 1s and 0s
    :param parts: Tuple of length of substrings
    :return: list of substrings
    """
    out = []
    packet = packet.replace(' ', '')
    for part_length in parts:
        out.append(packet[0:part_length])
        packet = packet[part_length:]
    out.append(packet)
    return out


def parse_packet(packet, address_length):
    """Split a packet into its fields and return them as tuple."""
    preamble, address, payload_length, pid, no_ack, rest = split_packet(packet=packet,
                                                                        parts=(8, 8 * address_length, 6, 2, 1))
    payload, crc = split_packet(packet=rest, parts=(int(payload_length, 2) * 8,))

    assert preamble in ('10101010', '01010101')
    assert len(crc) in (8, 16)

    return preamble, address, payload_length, pid, no_ack, payload, crc


def crc(bits, init=0xFF, polynomial=0x107, mode=8):
    """Calculate the crc value for the polynomial initialized with 0xFF)
    :param mode: 8 or 16 bit crc
    :param init: Initial value of the crc registers
    :param polynomial: standard is 0x107 = 0b100000111 = x^8+x^2+x^1+1  Note: The code does the same for 0x07
    :param bits: String of 1s and 0s
    :return:
    """
    crc = init
    max_crc_value = (1 << mode) - 1  # e.g. 0xFF for mode 8bit-crc
    for bit in bits:
        bit = int(bit, 2)
        crc <<= 1
        if (crc >> mode) ^ bit:  # top most lfsr bit xor current data bit
            crc ^= polynomial
        crc &= max_crc_value  # trim the crc to reject carry over bits
    return crc


if __name__ == '__main__':
    """Split test packets into their fields and validate the crc."""
    packets = \
        (
            '10101010 11101110 00000011 00001000 00001011 01000111 000100 00 0 10101010 10101010 10101010 10101010 10010010',
            '10101010 11101110 00000011 00001000 00001011 01000111 000100 01 0 10101010 10101010 10101010 10101010 01010110',
            '10101010 11101110 00000011 00001000 00001011 01000111 000100 10 0 10101010 10101010 10101010 10101010 00011101',
            '10101010 11101110 00000011 00001000 00001011 01000111 000100 11 0 10101010 10101010 10101010 10101010 11011001',
        )
    for packet in packets:
        preamble, address, payload_length, pid, no_ack, payload, crc_received = \
            parse_packet(packet=packet, address_length=5)

        crc_received = hex(int(crc_received, 2))  # convert into hex format

        print(packet)
        print('\n'.join((
            f'preamble: {preamble}',
            f'address: {bin2hex(address)}',
            f'payload length: {int(payload_length, 2)}',
            f'pid: {int(pid, 2)}',
            f'no_ack: {int(no_ack, 2) == 1}',
            f'payload: {bin2hex(payload)}',
            f'crc: {crc_received}')))
        crc_calculated = hex(crc(address + payload_length + pid + no_ack + payload, mode=8))
        if crc_received == crc_calculated:
            print('CRC is valid!')
        else:
            print(f'CRC mismatch! Calculated CRC is f{crc_calculated}.')

        print('---------')